Medical devices, such as cardiac pacemakers and implantable cardioverter defibrillators (ICDs), provide therapeutic electrical stimulation to a heart of a patient via electrodes carried by one or more medical electrical leads and/or electrodes on a housing of the medical device. The electrical stimulation may include signals such as pacing pulses or cardioversion or defibrillation shocks. In some cases, a medical device may sense cardiac electrical signals attendant to the intrinsic or pacing-evoked depolarizations of the heart and control delivery of stimulation signals to the heart based on sensed cardiac electrical signals.
Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm of the heart. For example, an ICD may deliver pacing pulses to the heart of the patient upon detecting bradycardia or tachycardia or deliver cardioversion or defibrillation shocks to the heart upon detecting tachycardia or fibrillation. The ICD may sense the cardiac electrical signals in a heart chamber and deliver electrical stimulation therapies to the heart chamber using electrodes carried by transvenous medical electrical leads. Cardiac signals sensed within the heart generally have a high signal strength and quality for reliably sensing cardiac electrical events, such as R-waves. In other examples, a non-transvenous lead may be coupled to the ICD, in which case cardiac signal sensing presents new challenges in accurately sensing cardiac electrical events and properly detecting and discriminating between different types of cardiac arrhythmias.
Proper detection and discrimination of different tachyarrhythmias is important in automatically selecting and delivering an effective electrical stimulation therapy by an implantable medical device system and avoiding unnecessary therapies. For example, a supraventricular tachyarrhythmia originates in the upper, atrial heart chambers and is conducted to the lower, ventricular heart chambers. A supraventricular tachyarrhythmia (SVT) is generally not successfully terminated by delivering electrical stimulation therapy to the ventricles because the heart rhythm is arising from the upper heart chambers. A ventricular tachyarrhythmia that originates in the lower, ventricular heart chambers, on the other hand, generally can be successfully treated by delivering electrical stimulation therapies to the ventricles to terminate the abnormal ventricular rhythm. Accordingly, discrimination of supraventricular tachyarrhythmia that originates in the upper heart chambers from ventricular tachyarrhythmia that originates in the lower heart chambers allows for appropriate therapy selection and delivery while avoiding unnecessary or potentially ineffective electrical stimulation therapy from being delivered to the patient's heart.